The present invention relates to improvements in presses for mechanically pressing water from a traveling web in a paper machine, and more particularly of the press of the type known as an extended nip press. In an extended nip press, the length of a single nip in the machine direction extends substantially longer than the nip of a conventional type formed between two mating rolls. More particularly, the invention relates to improvements in means for containing lubricant which is used to provide a wedge of lubricating film between the shoe and belt in an extended nip press of the type which employs a concave shoe urged toward a supporting roll to form a nip with the shoe urged toward the nip and the shoe supported in a manner so that it will be self-positioning to maintain the wedge of lubricant.
Extended nip presses of this type have been formed in various arrangements with one type of arrangement disclosed and illustrated in Justus U.S. Pat. No. 3,783,097. In a press of this type in a high speed pressing operation, oil is employed as a lubricant between the shoe and belt. Oil will form a wedge that will not break down under normal operating speeds. This type of press is particularly advantageous in extending the time to which a sheet of paper is exposed to pressing pressure within the nip. It is also advantageous in that the invention of this press permits control of the pressure profile within the nip. Optimum pressing and variations in pressure profile can be attained by varying the location of the shoe relative to its support and by changing the curvature of the shoe. Advantages in pressing with this type of press, known as high impulse pressing, include substantially increased dewatering as contrasted with a two roll press, improvement in paper strength and uniformity of the paper web in a cross-machine direction. The combination of these advantages and the magnitude of the advantages make this advance in the art of this particular type of extended nip press of the Justus U.S. Pat. No. 3,783,097 one of the outstanding and most important advances in the papermaking field in many years.
In operation of this type of shoe press, at high operating speeds, a mist of oil tends to be generated as the belt passes the shoe and develops the wedge of lubricant. Various efforts have been made to control the oil and contain it. One arrangement has been to provide seals around the shoe, and an example of this is illustrated in U.S. Pat. No. 4,398,997, Cronin.
Another example of an approach in containing the oil has been to place end walls in a looped annular belt, and this is being done commercially by Voith GmbH of Heidenheim, Germany. The Voith arrangement causes considerable problems in that the end walls are circular with the belt following its natural circular looped shape. The belt must curve in an arc opposite to the direction of the looped shape while passing through the nip formed between the shoe and the roll. At the ends of the nip beyond the end of the shoe, the belt is held by the circular end walls and is compelled to flex in a double plane direction. That is, the ends of the belt tend to continue in the natural circular shape and are held in this natural circular shape by the end walls which are rigidly connected to the end of the belt. This causes continual flexure of the belt in a double plane opposite the end of the shoe and incurs failure of the belt by rupturing and breaking due to the continual double plane flexing.
It is accordingly an object of the present invention to provide a method and apparatus for containing lubricant within a looped belt of a shoe type press without shortening the life of the belt due to its flexing in a double plane at the ends of the nip.
A further object of the invention is to provide an improved shoe type press utilizing an improved method and apparatus for containing oil within a looped belt of the press by providing an end wall which is connected to the belt in such a manner that it permits the belt to follow the single plane of curvature of the nip beyond the ends of the nip thereby avoiding the disadvantages of an end wall structure such as that employed by the aformentioned Voith arrangement.
Other objects, advantages and features, as well as equivalent methods and structures which are intended to be covered herein, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: